Technics for head-mounted display devices (hereinafter referred to as HMDs) in which a holographic element or a diffractive optical element is used are existed. In the HMD, image light emitted from a liquid-crystal device or the like is guided along a given optical path using a light guide element, such as that described above, so as to be viewed by the user.
For example, Patent Document 1 describes an example of HMDs which employs a reflective volume holographic element. Patent Document 2 describes an example of HMDs which has been configured so that an image displayed on a display element is optically enlarged to form an enlarged virtual image, which is viewed by the user. In Patent Document 2 is further shown an example in which in order to widen the viewing angle of the virtual image which is the image to be viewed by the user, the light guide member is constituted of a glass material having a refractive index for the wavelength of the image of 1.6 or higher.
As techniques relevant to the present invention, techniques for reducing the temperature dependence of diffraction angle are described in Patent Documents 3 to 5.